A secondary battery having easy application to various product groups and good electric characteristics such as high energy density is widely used not only for portable devices but also for electric vehicles (EV) or hybrid electric vehicles (HEV), which are driven by an electric driving source. The secondary battery receives great attention as a new energy source for natural-friendly property and enhanced energy efficiency since it gives a primary advantage of greatly reducing the use of fossil fuel and also does not generate byproducts at all in use.
Currently, lithium ion batteries, lithium polymer batteries, nickel cadmium batteries, nickel hydride batteries, nickel zinc batteries, or the like are widely used as secondary batteries. Such a unit secondary battery cell has an operating voltage of about 2.5V to 4.2V. Therefore, if higher output voltage is demanded, a plurality of secondary battery cells may be connected in series to configure a battery pack. In addition, according to a charging/discharging capacity demanded at the battery pack, a plurality of secondary battery cells may be connected in parallel to configure the battery pack. Therefore, the number of secondary battery cells included the battery pack may be set variously depending on the demanded output voltage or charging/discharging capacity.
Meanwhile, when a battery pack is configured by connecting a plurality of secondary battery cells in series or in parallel, in general cases, a battery module including at least two secondary battery cells is configured first, then a battery module assembly is configured using a plurality of battery modules, and then other components are added to the battery module assembly to configure a battery pack.
The battery pack includes a battery module assembly having a plurality of secondary batteries, an upper plate provided at an upper portion of the battery module assembly, a lower plate provided at a lower portion of the battery module assembly, and a coupling member for coupling the upper plate and the lower plate. The upper plate and the lower plate may fix the outer appearance of the battery module assembly and protect the battery module assembly against external impacts.
However, if the secondary battery included in the battery module assembly is a pouch-type lithium polymer secondary battery, the internal electrolyte may be decomposed due to side reactions caused by repeated charging and discharging, which may generate gas. At this time, the generated gas may deform the outer appearance of the secondary battery cell, which is called ‘swelling phenomenon’.
Due to the swelling phenomenon, in charging, the battery module assembly is expanded to increase the pressure applied to the upper and lower plates by the battery module assembly. If so, other structures such as the coupling member, provided at the battery pack, may also be deformed.
In order to prevent the battery pack from having deteriorated performance due to such a swelling phenomenon, when the upper and lower plates are designed, their rigidity is set in consideration of swelling phenomenon. However, just with the rigidity design in consideration of allowance, material, thickness, shape or the like of the upper and lower plates, there is a limit in preventing the performance deterioration of the battery pack caused by the swelling phenomenon.